Research has been undertaken to convert biomass into transportation fuels over the past three decades. A main objective of research done during the 1990s was to develop enzymes that could hydrolyze cellulose into sugars. This enzymatic hydrolysis of pure cellulose is a slow but well established process. However, biomass does not yield pure cellulose efficiently. Some form of pretreatment is required to make the biomass amenable to efficient enzymatic hydrolysis. Although pretreatment technologies exist, none is suitable for economic production of fuels and chemicals from biomass. Brodeur, et al. (2011) Enzyme Research Article ID 787532, 17 pages.
Lignocellulose is the major structural component of plants and comprises cellulose, hemicellulose, and lignin. In lignocellulosic biomass, crystalline cellulose fibrils are embedded in a less well-organized hemicellulose matrix which, in turn, is surrounded by an outer lignin seal. Lignocellulosic biomass is an attractive feed-stock because it is an abundant, domestic, renewable source that can be converted to liquid transportation fuels, chemicals and polymers. The major constituents of lignocellulose are: (1) hemicellulose (20-30%), an amorphous polymer of five and six carbon sugars; (2) lignin (5-30%), a highly cross-linked polymer of phenolic compounds; and (3) cellulose (30-40%), a highly crystalline polymer of cellobiose (a glucose dimer). Cellulose and hemicellulose, when hydrolyzed into their monomeric sugars, can be converted into ethanol fuel through well established fermentation technologies. These sugars also form the feedstocks for production of a variety of chemicals and polymers. The lignin may also be recovered for use in the production of a variety of chemicals or used a fuel. The complex structure of biomass requires proper treatment to enable efficient hydrolysis (e.g., saccharification) of cellulose and hemicellulose components into their constituent sugars. Current treatment approaches suffer from slow reaction rates of cellulose hydrolysis (e.g., using the enzyme cellulase) and low sugar yields. Wyman, et al. (2005) Bioresource Technology 96: 1959-1966).
Contacting lingocellulosic biomass with hydrolyzing enzymes generally results in cellulose hydrolysis yields that are less than 20% of predicted results. Hence, some “pretreatment” of the biomass is invariably carried out prior to attempting the enzymatic hydrolysis of the cellulose and hemicellulose in the biomass. Pretreatment refers to a process that converts lignocellulosic biomass from its native form, in which it is recalcitrant to cellulase enzyme systems, into a form for which cellulose hydrolysis is effective. Compared to untreated biomass, effectively pretreated lignocellulosic materials are characterized by an increased surface area (porosity) accessible to cellulase enzymes, and solubilization or redistribution of lignin. Increased porosity results mainly from a combination of disruption of cellulose crystallinity, hemicellulose disruption/solubilization, and lignin redistribution and/or solubilization. The relative effectiveness in accomplishing at least some of these factors differs greatly among different existing pretreatment processes. These include dilute acid, steam explosion, hydrothermal processes, “organosolv” processes involving organic solvents in an aqueous medium, ammonia fiber explosion (AFEX), strong alkali processes using a base (e.g., ammonia, NaOH or lime), and highly-concentrated phosphoric acid treatment. Many of these methods do not disrupt cellulose crystallinity, an attribute vital to achieving rapid cellulose digestibility. Also, some of these methods are not amenable for efficient recovery of the chemicals employed in the pretreatment.
Ionic liquid pretreatment technique is effective in disrupting the recalcitrance of biomass for subsequent conversion to value added products. Anantharam, et al. (2006) “Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment step.” Biotechnol. and Bioengg. 95(5): 904-910; Anantharam, et al. (2007) “Mitigation of cellulose recalcitrance to Enzymatic hydrolysis by ionic liquid pretreatment.” Applied Biotechnol. and Bioengg 136-140: 407-421; Wang, et al. (2012) “Ionic Liquid Processing of Cellulose.” Chemical Society Reviews 41: 1519-1537; U.S. Pat. No. 7,674,608; and U.S. Pat. No. 8,030,030.
For commercial viability, the pretreatment of biomass should be conducted at high solids loadings (>20% w/w) to minimize the reactor size and process utility costs. However, the non-conducting/insulating characteristics pose significant heat and mass transfer limitations when process heating is done through jacketed tanks or other heated surfaces. Therefore, in these processes, at feed concentrations >20% (w/w), heat cannot penetrate uniformly and the slurries become thick, viscous, and non-uniformly wet. Viamajala, et al. Heat and Mass Transport in Processing of Lignocellulosic Biomass for Fuels and Chemicals, in Sustainable Biootechnology. Sources of Renewable Energy, O. V. Singh and S. P. Harvey, Editors. 2010, Springer: London, N.Y. This poses operational challenges in overcoming any localized heating zones or large heat gradients in the reactor, resulting in ineffective treatment of the feedstock.
Therefore, there is a need in the art for a method and system of treating biomass (e.g., lignocellulosic biomass) to prepare it for hydrolysis at high solids loadings and large scale to minimize reactor size and utility costs.